Field of the Invention
This description relates to a light emitting device package and a backlight unit using the same.
Discussion of the Related Art
A conventional light emitting diode is optically disposed with a dome-shaped lens, and light is limitedly distributed to within a predetermined region with respect to a central axis. If a light emitting diode is employed to manufacture a liquid crystal display (LCD) for backlight unit, one potential problem is that an even light characteristic cannot be obtained due to light emitting characteristic of the light emitting diode.
It implies that a considerable distance is needed to evenly combine light radiated from the light emitting diode, making it difficult to attain a uniform light characteristic from a thin backlight unit. In other words, a backlight unit using a light emitting diode suffers from a disadvantage of increasing the thickness of an LCD system.
FIG. 1 illustrates a light path from a lateral optical lens according to prior art, where a light emitting diode (LED. 10) is disposed inside a dome-shaped lens (20). The dome-shaped lens (20) is formed thereon with an inclined cone-shaped groove (21) and its side is formed with a V-shaped groove (22).
If light emitted from the LED 10 contacts a surface of the cone-shaped groove (21), the light is reflected from the inclined conical groove (21) to be radiated sidewise of the lens. If light contacts the V-shaped groove (22) of the lens (20), the light passes through the lens (20) to be radiated sideways of the lens.
In other words, an LED for laterally emitting (or side-emitting) light {hereinafter referred to as a lateral (side-emission type) LED} according to the prior art serves to laterally radiate light emitted from the LED (10), using a lateral optical lens.
Meanwhile, in the injection-molded lens (20), a region corresponding to an apex (22a) of the V-shaped groove (22) is formed with prominences and depressions (unevenness) if closely looked at (for example, in less than a millimeter unit), such that light of the LED (10) emitted from the region is not radiated sideways of the lens (10) but upwards of the lens (10).
FIG. 2 is a schematic perspective view of an LED package of FIG. 1, where the LED is bonded to a slug, and the slug is disposed at sides thereof with leads (31, 32) which are in turn electrically bonded to the LED.
Furthermore, the LED and the slug are molded by molding means in order to expose a light emitting surface of the LED and the leads (31,32), and the lens (20) of FIG. 1 encompassing the LED is bonded to the molding means.
FIG. 3 is a light emitting distribution table of an LED package according to the prior art, where it shows that a large amount of light is radiated sidewise of the package as indicated in ‘a’ and ‘b’ of the distribution table while a small amount of light is radiated through a center of the package as indicated in ‘c’, ‘d’ and ‘e’ of the distribution table.
FIG. 1 implies that although most of the light is emitted sideways of the lens, some of the light is radiated upwards of the lens. In other words, the LED package thus described cannot implement a perfect light emission to lateral surfaces, such that if it is to be used as a light source for a display, light is partially emitted from the light emitting diode relative to the center of the LED package, bringing about a problem of making an even planar light source.
To be more specific, the partial emission of light relative to the center of the LED package results in creation of so-called light irregularity, referred to as a hot spot phenomenon where spots are generated about a center of pixels displayed on a display, causing degradation of picture quality on the display. FIG. 4 illustrates in detail one of the causes generating the hot spots.
If a size of a light emitting diode (10) is extremely small, an amount of light emitted to a lateral surface of a conventional lens increases by being reflected from a surface (21) of the cone-shaped groove, but if the size of the light emitting diode (10) is extremely large, light (C) progressing at an angle less than a critical angle from the surface (21) of the cone-shaped groove exists to allow the light to be emitted from an upper surface of the lens, thereby generating the appearance of hot spots, because the surface (21) of the cone-shaped groove totally reflects only the light (A) advancing at an angle larger than the critical angle, out of the light irradiated from the light emitting diode (10).
At this time, light (B) progressing to lateral surfaces of the lens has nothing to do with hot spots, as shown in FIG. 4.
FIG. 5 illustrates a cross-sectional view of a light emitting diode packaged in a printed circuit board according to the prior art, where a plurality of lateral light emitting diode packages (50) are packaged in a printed circuit board (60). A printed circuit board packaged with lateral light emitting diode packages is employed for a backlight unit as depicted in FIG. 6.
FIG. 6 is a schematic cross-sectional view of a light emitting diode employed for a liquid crystal display (LCD) for backlight unit according to the prior art.
In order to address the problem of the light emitted to the center of the light emitting diode package, an LCD backlight unit is mounted with a hot spot baffle plate (80). In other words, an LCD backlight unit (90) is configured in such a manner that hot spot baffle plates (80) are mounted on each light emitting diode package (70) packaged in the printed circuit board (60), and a light guide plate (85) is positioned on an upper surface of the hot spot baffle plate (80), and an upper surface distanced from the light guide plate (85) is disposed with an LCD (95) to assemble the backlight unit (90) and the LCD 95.
However, there is a disadvantage in the backlight unit (90) thus constructed in that a plurality of light emitting diode packages (70) must be mounted thereon with hot spot baffle plates (80) called diverters to complicate the fabrication process.
There is another disadvantage in that if there is an erroneous arrangement of the hot spot baffle plates (80) on the plurality of light emitting diode packages (70), spots similar to the hot spots are generated on a screen of a display. Still another disadvantage is that the display panel increases in thickness as much as the hot spot baffle plate (80).